[PATCH 0/3] ipv4 metrics super-sharing

As I mentioned a few days ago, we can "super-share" the metrics
in the most common case, which is that there are not explicit
metrics set for a route.

This makes the metrics all zero, which means we can just alias
the pointer to dst_default_metrics.

Most importantly, this means we avoid atomics on the fib_info ref
count at dst destroy time.

Depending upon the configuration, this can also save a non-trivial
amount of memory in the routing tables.  Honestly, I doubt that
sites entertaining routing tables with hundreds of thousands of
routes have any non-zero metrics specified. :-)